docs: def ensure_loopback_device(path, size):
      '''
      Ensure a loopback device exists for a given backing file path and size.
      If it a loopback device is not mapped to file, a new one will be created.
  
      TODO: Confirm size of found loopback device.
  
      :returns: str: Full path to the ensured loopback device (eg, /dev/loop0)
      '''
      for d, f in six.iteritems(loopback_devices()):
          if f == path:
              return d
  
      if not os.path.exists(path):
          cmd = ['truncate', '--size', size, path]
          check_call(cmd)
  
      return create_loopback(path)